Process for producing acrylic synthetic fibers improved in the hydrophilicity

ABSTRACT

A PROCESS FOR MAKING ACRYLIC FIBERS MORE HYDROPHILIC BY WET-SPINNING AN ACRYLONITRILE POLYMER SOLUTION TO FORM A WET-GEL FIBER, STRETCHING THE WET-GEL FIBER, CROSSLINKING THE WET-GEL FIBER, HYDROLYZING THE CROSSLINKED WET-GEL FIBER OPTIONALLY TREATING THE HYDROLYZED CROSSLINKED WET-GEL FIBER WITH AN AQUEOUS SOLUTION OF AN AMMONIUM OR METAL SALT TO IMPROVE ANTI-STATIC POPERTIES, AND THEREAFTER DRYING THE FIBER.

United States Patent 3,733,386 PROCESS FOR PRODUCING ACRYLIC SYNTHETIC FIBERS IMPROVED IN THE HYDROPHILICITY Keitaro Shimoda, Takehiko Sumi, and Yoshiharu Mochida, Okayama, Japan, assignors to American Cyanamid Company, Stamford, Conn. N0 Drawing. Filed Apr. 13, 1971, fier. No. 133,709 Int. Cl. Dtllf 7/00 US. Cl. 264182 9 Claims ABSCT OF THE DISCLOSURE A process for making acrylic fibers more hydrophilic by wet-spinning an acrylonitrile polymer solution to form a wet-gel fiber, stretching the wet-gel fiber, crosslinking the Wet-gel fiber, hydrolyzing the crosslinked wet-gel fiber, optionally treating the hydrolyzed crosslinked wet-gel fiber with an aqueous solution of an ammonium or metal salt to improve anti-static properties, and thereafter drying the fiber.

This invention relates to processes for producing acrylic fibers of improved hydrophilic properties. More particularly, this invention relates to a process for producing acrylic fibers having an unusual balance of such physical properties as knot strength and elongation in combination with improved hydrophilic nature and antistatic activity. The property balance is otbained by crosslinking and hydrolyzing a spun stretched wet-gel acrylic fiber and subsequently drying the fiber to compact the fiber structure.

Synthetic fibers generally have excellent wash-and-wear properties because of their extreme hydrophobic nature. It is also known that such fibers have extensive uses arising from greater knot strength and elongaiton than natural fibers, which properties arise from high cohesive energy zones within the fiber structure.

However, when desirable, it is diflicult to produce low cohesive energy zones within the fiber structure. Accordingly it is extremely difiicult to improve the hydrophilic nature of such fiber and such difiiculty tends to restrict usage of such fibers where hydrophilic properties are desirable, such as in fabrication of underwear and bedding materials.

Numerous attempts have been made to improve the hydrophilic nature of such fibers so as to eliminate restrictions as to fiber uses. For example, a method involving the employment of a hydrophilic comonomer in preparation of a copolymer from which fibers are to be spun has been effective in improving hydrophilic properties of a yarn mixture containing fibers of the copolymer. Another method involves hydrolysis of the fiber-forming polymer to improve its hydrophilic nature. One such method involves treating previously compacted acrylic fibers With a crosslinking agent and subsequently hydrolyzing the crosslinked fibers, whereby fibers improved in dyeability and hydrophilic properties are obtained. The latter method, however, does not produce uniform crosslinking and hydrolysis throughout the fiber structure and, although dyeability and hydrophilic properties are improved to some extent, the improvements are non-uniform and do not satisfy the practical requirements desired. In this method, knot strength and elongation of the fibers are reduced to a point that adversely affects fiber utility. Thus, there continues to exist the need for a process whereby acrylic fibers can be improved with respect to hydrophilic properties while at the same time avoiding losses of desirable properties such as high knot strength and elongation.

In accordance with the present invention there is provided a process for obtaining acrylic fibers of improved hydrophilic properties which comprises treating a stretched swollen wet-gel state acrylic fiber with a crosslinking agent,

hydrolyzing the cross-linked Wet-gel fiber with a mineral acid, treating the hydrolyzed, crosslinked, wet-gel fiber with an aqueous solution of an ammonium or metal salt if desired, and then subjecting the fiber to drying to obtain a fiber of uniform and compact structure. The present invention provides hydrophilic properties increased to a greater extent than possible by prior art procedures while at the same time maintaining a favorable balance of such properties as knot strength, elongation, and Youngs modulus in hot water.

In carrying out the process of the present invention, it is essential that the fiber to be crosslinked be in the wet-gel state. Such requirement is necessary in order that the cohesive energy of the fiber in the radial direction be of low order and polymer modifications can be uniformly carried out within the fiber structure. More precisely, the fiber, which has been stretched and oriented, is in the wet-gel state and, as a result, contains at least 40% by weight of water, based on the weight of dry fiber. Without the content of Water, it is impossible to obtain a crosslinked fiber which is uniform throughout the fiber structure and to effect proper relaxation of the stretched fiber. To effect crosslinking of the stretched Wet-gel fiber, an aqueous solution containing one or more aldehyde compounds and one or more inorganic acids is employed. Treatment of the stretched wet-gel fiber with the aldehydeacid solution serves to relax the stretched fiber as well as to crosslink it and such treatment is preferably carried out at a temperature in the range of 60-100" C.

In effecting crosslinking while relaxing the stretched wet-gel fiber, the extent to which crosslinking occurs has a considerable effect on strength and elongation of the fiber finally obtained. Accordingly, in order to obtain a desirable balance between increased hydrophilic nature and other fiber properties, it is necessary to control the degree to which crosslinking occurs. A useful degree of crosslinking effected is that which produces a sodium thiocyanate swelling degree of less than 8, preferably less than 4. In the event the sodium thiocyanate swelling value, the determination of which is described subsequently, exceeds 8, part of the fiber-forming polymer rendered hydrophilic may dissolve out of the fiber structure, the Weight of the fiber may be reduced thus causing a structure-thinning phenomenon, and the knot strength of the eventual fiber may be unsatisfactory.

As indicated, the crosslinking composition is an aqueous solution of an aldehyde and an inorganic acid. Useful aldehydes include formaldehyde, glyoxal, benzene paradialdehyde, and the like. Compounds which are capable of generating an aldehyde under the conditions of treatment are also eifective and include such compounds as described in US. Pat. No. 2,889,192, for example. As inorganic acids are included such acids as nitric, sulfuric, perchloric, hydrochloric, phosphoric, and the like.

In carrying out the crosslinking step, the sodium thiocyanate swelling value obtained will vary widely depending upon the nature and amount of monomers present in the fiber-forming polymer, the concentrations of aldehyde and acid employed, and the temperature and duration, of the crosslinking reaction. For example, using an aqueous solution of formaldehyde and sulfuric acid, it is possible to impart an effective degree of crosslinking by treating the fiber for at least 30 seconds at a temperature in the range of 60100 C. When the composition contains 40-65% by weight of acid and 3-20% by Weight of aldehyde, based on the total weight of treating solution.

The sodium thiocyanate swelling value indicates the degree of crosslinking, the smaller the value, the greater is the degree of crosslinking. This value is determined by taking a small sample of fiber subjected to the crosslinking reaction and determining its weight, W,,. The sample is then treated at 70 C. for 40 minutes with an aqueous solution containing 60% sodium thiocyanate, based on the weight of solution. The sodium thiocyanate solution is then removed by use of filter paper and the fiber again weighed to obtain W The sodium thiocyanate swelling value is equal to 0.l39+0.775 W /W Following crosslinking of the stretched wet-gel fiber, the fiber preferably is next washed with water and then hydrolyzed by the action of 'an inorganic acid. Such hydrolysis causes hydrolyzable side groups of the fiber-forming polymers to become hydrophilic. Although the hydrolysis can be carried out in a single stage, it is generally preferred to employ two stages involving different conditions. In a first stage, less than about half of the hydrolyzable side groups of the fiber-forming polymer may be converted from, for example, cyano to amido groups to produce the desired increase in hydrophilic nature while maintaining a desirable level of knot strength, which tends to decrease with increasing hydrolysis. Then in a second hydrolysis stage, different in conditions from the first stage, at least a portion of the amido groups produced in the first stage is converted to carboxyl groups. The' carboxyl groups produced serve as effective dye sites for basic dyestulfs and greatly increase the extent to which level dyeability can be obtained in the final fiber. It is, of course, possible by proper selection of acid concentration, temperature, and treating time to obtain the desired extent of hydrolysis in a single stage. It is preferable to achieve as a lower limit hydrolysis of at least 15% of the cyano groups to amido groups so as to provide a significant increase in the hydrophilic nature of the final fiber.

Acids useful in the hydrolysis step are of the same type specified with respect to the crosslinking step. Proper selection of the acid concentration, temperature, and time of treatment in carrying out the hydrolysis can lead to significant improvement in the hydrophilic nature of the final fiber and, at the same time, can provide increased level dyeability while maintaining desirable values of knot strength, elongation and Youngs modulus.

The composition of aqueous acid solution used in the hydrolysis step will also vary depending upon the specific inorganic acid employed. When, for example, sulfuric acid is employed in a two-stage hydrolysis step, suitable conditions for the first stage employ 'an acid concentration of 70 to 85%, by weight, based on the weight of the hydrolysis bath, a temperature not exceeding 80 C., and a treatment time of at least 15 seconds, while in the second stage suitable conditions are an acid concentration of to 50% by weight based on the weight of the hydrolysis bath, a temperature in the range of 50 to 100 C., and a treatment time of at least 30 seconds. After the hydrolysis step is complete, the gel-stage fiber thus processed preferably is washed with water and then compacted by drying, the uniform structure being maintained upon subsequent processing.

In the present invention, by crosslinking and heatrelaxing a swollen wet-gel fiber containing sufficient water to have cohesive energy of low order in the radial direction, it is possible to obtain uniform crosslinking over the entire fiber structure, and because the nature of the crosslinks introduced does not lead to a three-dimensional structure as is the case when crosslinking is accomplished after the wet-gel fiber has been dried, it is possible to impart uniform property modifications over the entire crosssection of the fiber. Thus, the present invention provides a novel process wherein a fiber structure reduced in cohesive energy in the radial direction is crosslinked while being heat-relaxed, and then hydrolyzed and dried, whereby there is obtained a fiber of highly desirable hydrophilic nature While other fiber properties are maintained at a high level.

On the contrary, when a fiber is spun, stretched, and dried so as to be compacted subsequent crosslinking and hydrolysis of the fiber leads to a fiber structure which is non-uniform in modification from surface to core, with crosslinked, hydrolyzed, and unmodified areas arranged respectively as the core is approached. The resulting fiber cannot be effectively heat-relaxed and physical properties, such as knot strength, are excessively reduced.

In the present invention, by employing a fiber in the swollen Wet-gel state wherein the cohesive energy in the radial direction is of low order, it is possible to select a wide range of conditions for carrying out the crosslinking and hydrolyzing steps. This wide selection is not possible when the fiber has been compacted prior to crosslinking and hydrolysis and the limited conditions that can be employed to achieve a practical modification of fiber properties are such as to destroy other fiber properties, for example, the concentration of acid required in the hydrolysis step is sufficient substantially to dissolve the fiber. It can be appreciated that in the present invention, for example, the Wide range of conditions useful in carrying out the crosslinking and hydrolysis steps permits control over the extent to which level dyeability properties can be improved in addition to controlling improvements in bydrophilic properties.

The fiber hydrolyzed according to the present invention may be treated, if desired, with an aqueous solution of one or more salts in which the cation is of a metal selected from Groups I, II, and HI of Mendeleevs Periodic Table or an ammonium cation, whereby the hydrogen ion produced by hydrolysis is replaced by the cation selected. In this manner the hydrophilic nature of the fiber is further improved and anti-static properties are conferred to the fiber. It can be appreciated that acrylic fibers used for underwear and bedding materials are benefited by improvement in both hydrophilic and antistatic properties. Thus, treatment of the fiber of the present invention with a metal or ammonium salt solution further enhances the benefits obtained as a result of the present invention.

Treatment of the fiber with an aqueous solution of salt may be carried out at any time after a carboxylic acid group has been produced by hydrolysis. However, it is preferable to carry out treatment with salt while the fiber remains in the undried state, i.e. prior to compacting.

As the treating solutions, there can be used those containing such salts as, for example, sodium sulfate, sodium carbonate, sodium chloride, potassium sulfate, potassium carbonate, magnesium sulfate, magnesium carbonate, ammonium sulfate, and ammonium carbonate.

The acrylic fiber to be modified in accordance with the present invention is one in which the fiber-forming polymer contains at least about by weight of acrylonitrile. As monomers that can be copolymerized with acrylonitrile to form useful polymers are included: amides such as acrylamide, methacrylamide, allylamide, and tat-methylene glutaramide; hydroxyl containing monomers such as allyl alcohol, B-hydroxyethyl methacrylate, and B-hydroxypropyl acrylate; such esters as methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, methoxyethyl acrylate, phenyl acrylate, cyclohexyl acrylate, dimethylaminoethyl acrylate, and corresponding esters of methacrylic acid; alkyl chain substituted derivatives and nitrogen substituted derivatives of acrylamide and methacrylamide; unsaturated ketones such as methylvinyl ketone, phenylvinyl ketone, and methyl isopropenyl ketone; vinyl carboxylates such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; esters of a, 3-ethylenically unsaturated acids such as fumaric, citracom'c, mesaconic, and aconitic acids; N-alkyl maleimides; N-vinyl carbazol and N-vinyl succinimide; N-vinyl phthalimide; vinyl ether; vinyl pyridines such as 2-vinyl pyridine, 4-vinyl pyridine, and Z-methyl, 5-viny1 pyridine; styrene and alkyl substituted derivatives thereof; vinyl chloride; vinylidene chloride; and vinylidene cyanide.

As the solvent useful in preparing a spinning solution of the fiber-forming polymer, there may be used a concentrated solution of inorganic salt, concentrated nitric acid, or an organic solvent. Concentrated aqueous solutrons of inorganic salts useful are those of such as sodium hiocyanate, potassium thiocyanate, ammo i m thiocyaa nate, calcium thiocyanate, zinc chloride, lithium chloride, and mixtures thereof. As organic solvents, for example, dimethyl formamide and dimethyl sulfoxide are useful. As coagulant for the spinning solution, there may be employed water, aqueous solutions of the abovementioned organic solvents at a concentration of 20 to 70% by weight thereof.

The invention will be further illustrated by the examples which follow in which all parts and percentages are by weight unless otherwise specifically stated.

In the examples which follow various fiber property values are reported. The various property evaluations were carried out as described below.

Hygroscopicity (moisture regain).-About 2 grams of test fiber are predried for one hour at 80 C. and then stored at 20i2 C. and relative humidity of 65:2% for M hours, whereupon the weight is determined and designated weight A. The fiber is then dried for 20 hours at 60 C. and at a pressure of 50 millimeters of mercury, absolute. The fiber is again weighed and the weight designated B. Hygroscopicity, H, is then calculated from the expression:

A-B H= X100 Surface resistivity (antistatic property) .'Ihe electrical ting bath of 12% aqueous sodium thiocyanate maintained at a temperature of 3 C. The spun fiber was then washed in water, stretched at a stretch ratio of 2.5 in the washing bath and then further stretched at a stretch ratio of 4.0 in boiling water. The swollen wet-gel fiber was then treated with an aqueous solution containing 50% sulfuric acid and 10% formaldehyde at 95 C. for 10 minutes to effect crosslinking and heat-relaxation while maintaining the swollen wet-gel state. The degree of crosslinking effected resulted in a sodium thiocyanate value of 4. The fiber was then divided into seven portions and one portion run in accordance with each of the hydrolysis conditions specified in Table I. Following hydrolysis the fibers were water-rinsed and then dried to produce a compact structure, except in the case of Run 3. In Run 3, the fiber after hydrolysis and water-rinsing was treated with an aqueous solution of sodium carbonate of pH 11 at 100 C. for 1 minute and then subjected to drying.

From the data given in Table I, it can be seen that the hydrophilic properties of the fibers can be greatly improved while maintaining a favorable balance of other fiber properties. The reduced electrical resistance of the fiber of Run 3 as a result of treatment with sodium carbonate is evident. In each instance, a dyeing test indicated complete penetration of the fiber cross-section by the basic dyestuif.

TABLE I.-HYDROLYSIS XIQINDI'IIONS AND FIBER PROPERTIES OF ACRYLIC FIBERS CROSSLINKED D HYDROLYZED WHILE SWOLLEN WET-GEL STATE Fiber r erties Primary hydrolysis Secondary hydrolysis p p Moisture Surface H 80 H2804, regain, resist, Yarn Knot Youngs percent 0. Mm. percent C. Mm. percent ohms strength 1 strength 1 modulus 1 80 10 7.4 6X10 3.0 1.86 80 10 6 1X10 2.8 1. 86 i? 80 10. 2 3X10 2. 2 1 78 (J, 41 70 100 6.1 8X10" 2. 4 2. 0 0. 37 85 30 14. 3 6X10 1. 2 1. 1 0. 36 90 30 2 3x10 0. 7 0.6 0. 22 8 40 9 10 0.8 0. 7 0.

l Grams per denier.

resistance of the surface of a fiber stored for 16 to 20 hours at a temperature of 20i2 C. and relative humidity of 651-2% is measured using a Textrome resistance meter, Model (ER-54, manufactured by Central Electronic Industrial Company, Ltd., Japan.

Youngs modulus, E .-A thermostatic tank containing water maintained at 95 C. is connected to a constant velocity elongation tester (Tensilon UTM III, manufactured by Oriental Measuring Instrument Company, Ltd., Japan). A test fiber sample of millimeter length is elongated at a velocity of 30 millimeters per minute, i.e. 100% per minute, and the load is detected by a load cell (TLU of 0.03 kilogram manufactured by Oriental Measuring Instrument Company, Ltd., Japan) of a resistance wire strain meter type. The meter records the load, a loadelongation curve results, and the Youngs modulus is determined from the initial gradient.

Dyeability.-A test fiber is immersed in a dyebath containing 3.4% on the weight of the fiber of C. I. Basic Blue. The pH of the dyebath is adjusted to 7 with sodium acetate and dyeing is effected at C. for 30 minutes. The dyed fiber in cross-section is then observed by use of a microscope to determine diifusion of dyestuif.

EXAMPLE 1 As fiber-forming polymer, there was employed a copolymer containing 90% acrylonitrile and 10% methyl acrylate. The polymer was prepared as a spinning solution of 11% polymer concentration in a polymer solvent which was an aqueous solution of 46% sodium thiocyamate. The polymer solution was wet-spun into a coagula- COMPARATIVE EXAMPLE A Following the procedure of Example 1, fibers were prepared repeating all essential steps up to and including the stretching in boiling water. In this example, instead of crosslinking and hydrolyzing the swollen wet-gel fiber, the fiber was first dried to compact the fiber structure and then heat-relaxed in saturated steam at 127 C. and dried.

The dried fiber was divided into five portions and then subjected to the crosslinking and hydrolyzing conditions given in Table II. After hydroly-zing, the fibers were water-rinsed and dried. Fiber properties are also given in Table II.

From the data of Table II, it is seen that it is not possible to obtain desirable increases in moisture regain without causing excessive losses in knot strength when the fiber is compacted prior to the crosslinking and hydrolyzing steps. In Runs a and b, the fibers were partially dissolved during the hydrolysis step, with low acid usage in hydrolysis producing a negligible eifect on moisture regain and high acid usage destroying fiber utility. In Runs c and d, where higher acid concentration was employed in crosslinking, the fibers did not dissolve during the hydrolysis step, but increased moisture regain was efiected at excessive losses in knot strength. In Run e, where the highest acid concentration was employed during crosslinking, the fiber became powdery and could not be further processed. The fibers of Runs a, c, and d, when subjected to the dyeing test, showed dyeing of only the outer periphery of the fiber cross-section.

TABLE II.CROSSLINKING AND HYDROLYZING CONDITIONS OF ACRYLIC FIBERS DRIED PRIOR TO SUBJECTION TO SUCH CONDITIONS AND RESULTING FIBER PROPERTIES Fiber properties crosslinking treatment HydrolyZing treatment Moisture H 804, HOIIO, H 504, regain, Knot percent percent C. Time percent C. Time percent strength 1 1 100 2 65 +90 1 2. 1 0. 9 l 100 2 80 10 10 l 100 2 05 l 5. 0 0. 1 100 2 80 10 6. 4 0. 4 1 100 2 COMPARATIVE EXAMPLE B the wet-gel fiber with an aldehyde in the presence of an The procedure of Example 1 was followed in preparing a fiber up to and including the stretching in boiling water. In this example, however, the swollen wet-gel fiber Was immediately subjected to hydrolysis in an aqueous 80% sulfuric acid solution at 10 C. for 10 minutes without subjecting the swollen wet-gel fiber to a preliminary crosslinking reaction. During the hydrolysis step, the fiber dissolved and thus was lost.

EXAMPLE 2 A fiber-forming polymer containing 90% acrylonitrile and 10% acrylamide was employed. A spinning solution containing 11% polymer Was prepared by dissolving the polymer in an aqueous solution of 46% sodium thiocyanate. The spinning solution was spun into a fiber following the conditions of Example 1 up to and including stretching in boiling water. Cross-linking, with heatrelaxation, was then effected by treating the swollen wetgel fiber with an aqueous solution containing 40% sulfuric acid and 10% formaldehyde for 10 minutes at 95 C. to obtain a sodium thiocyanate swelling value of 4. The crosslinked fiber was then water-rinsed and subjected to a first hydrolysis stage in aqueous 77% sulfuric acid for 10 minutes at C. and to a second hydrolysis stage in aqueous 40% sulfuric acid for minutes at 95 C. The hydrolyzed fiber was then water-rinsed and treated with an aqueous solution of sodium carbonate at pH 9 for 1 minute at 100 C. The fiber was then water rinsed and dried to compact its structure. The dried fiber exhibited a moisture regain of 8.2%, yarn strength of 2.3 grams per denier, knot strength of 1.8 grams per denier and a Youngs modulus of 0.46 gram per denier. The fiber upon subjection to the dyeing test showed uniform penetration of dyestuif throughout the cross-section of the fiber.

What is claimed is:

1. A process for producing acrylic fibers of improved hydrophilic properties which comprises wet-spinning a solution of an acrylonitrile polymer of at least 80% by weight of acrylonitrile and the balance of a monomer copolymerizable therewith to form a wet-gel fiber containing at least by Weight of water based on the weight of the dry fiber, stretching the wet-gel fiber, crosslinking inorganic acid to a sodium thiocyanate swelling value of less than about 8, hydrolyzing the cross-linked wet-gel iglger with an inorganic acid, and thereafter drying the 2. The process of claim 1 wherein subsequent to hydrolysis and prior to drying, the fiber is treated with an aqueous solution of an ammonium salt or a metal salt, said metal being selected from Groups I, II, or III of Mendeleevs Periodic Table.

3. The process of claim 1 wherein said crosslinking step is carried out to a point which is sufiicient to produce a sodium thiocyanate swelling value of less than about 4.

4. The process of claim 1 wherein said crosslinking step is carried out at a temperature in the range of to C.

5. The process of claim 1 wherein said hydrolysis step is carried out to the extent that less than half of the hydrolyzable groups of the fiber-forming polymer become hydrophilic.

6. The process of claim 1 wherein the hydrolysis step is carried out in stages.

7. The process of claim 6 wherein hydrolysis in a first stage is sufiicient to convert 15% of the hydrolyzable groups of the fiber-formin polymer to hydrophilic groups.

8. The process of claim 1 wherein said crosslinking step is carried out by use of an aqueous solution of sulfuric acid and formaldehyde.

9. The process of claim 2 wherein said salt is sodium carbonate.

References Cited UNITED STATES PATENTS 2,933,460 4/1960 Richter et al. 264342 RE 3,055,729 9/1962 Richter et a1 264-184 3,140,265 7/1964 Richter et al. 2602.l 3,233,026 2/1966 Richter et al. 264-178 F 3,399,177 8/1968 Reeder et al. 264-182 3,622,658 11/1971 Nakagawa 264-78 JAY H. WOO, Primary Examiner US. Cl. X.R. 

